The present invention relates to a steel cord adapted for reinforcement of a protection ply in a tire. Conveniently only one protection ply is provided per tire, but tires with more than one protection ply are not excluded.
The protection ply in a tire is the outermost ply in a tire and is the ply which lies closest to the tread and thus to the surface. As a direct result of its position in a tire and as its name says, a protection ply fulfills; a front line function in the protection of a tire: every unevenness and every roughness on the roads are first felt and taken up by the protection ply. Consequently particular requirements are put on cords reinforcing these protection plies.
First of all, the cords must have a high corrosion resistance, since moisture that is able to penetrate via cracks in the tread is most likely to arrive first at the protection ply. Full rubber penetration is a way to slow down the corrosion attack on steel cords. Secondly, the cords must have a high elongation in rubber before they break.
Thirdly, since the cords are not only subjected to elongation but also to compression, they must have a good compression behavior, which means that their deformation at the buckling point or at the point of instability must be relatively high, e.g. above 3%, or preferably above 4%.
As a fourth requirement, the cords must be low-cost.
The prior art has already provided a number of steel cords specially adapted for the reinforcement of protection plies, but no such cord fulfilled the above four requirements to a sufficient degree.
A first type of known steel cords for the reinforcement of protection plies are the so-called high-elongation (HE) cords, such as a 3xc3x977xc3x970.22 or a 4xc3x974xc3x970.22. These are cords comprising a number of strands which are arranged in a Lang""s lay configuration, which means that the direction of twist is the same in the strands as in the cord (SS or ZZ). The strands are loosely associated and movable relative to each other in order to give the final cord a high elongation at fracture (e.g. above 5%). This elongation is an elongation measured on the cord as such, not embedded in rubber. Due to the fact, however, that this elongation is mainly of a structural nature, a main part of this elongation gets lost once the cord is embedded in rubber: a sharp drop from above 6% to below 3% is not an exception. These cords have also other drawbacks: they do not allow rubber to penetrate inside the cord and they are not low-cost due to their relatively thin filaments and to their multi-strand character which necessitates two separate twisting steps.
A second type of known steel cords for the reinforcement of protection plies are the so-called elongation (E) cords. An example of an elongation cord is a 4xc3x972xc3x970.35 cord. Just as a high-elongation cord, an elongation cord is also a cord with multiple strands arranged in a Lang""s lay configuration (SS or ZZ). The elongation at fracture of the cord as such, i.e. not embedded in rubber, ranges from 4% to 6%. Here again, however, the elongation at fracture falls down to about 2% to 3% once embedded in rubber. An elongation cord also still necessitates two separate twisting steps.
It is an object of the present invention to provide a steel cord which is suitable for the reinforcement of a protection ply of a tire, i.e. a steel cord with a full rubber penetration, a good compression behavior, a high elongation in rubber and which is low cost.
According to the invention there is provided a steel cord adapted for reinforcement of a protection ply in a tire. The steel cord has under compression in rubber a deformation wk at instability of at least 3%, preferably at least 4%. The steel cord comprises steel filaments of a pearlitic structure. The steel cord is stress-relieved so that its total elongation at rupture in rubber exceeds 3.5%, preferably at least 4% and most preferably at least 5%.
Preferably the steel cord has such a cord structure that when it is subjected to an increasing tensile load only linear contacts are produced between the individual steel filaments. The reason is that with such steel cords the above-mentioned stress-relieving increases the total elongation at rupture in rubber relatively easily above 3.5% and even above 4%, whereas for other steel cords where tensile loads create point contacts between the individual steel filaments, it is more difficult or in some cases even impossible to reach the 4% level.
For reason of obtaining a determined level of breaking load, the diameter of the individual filaments preferably exceeds 0.30 mm, most preferably 0.35 mm, e.g. 0.38 mm or 0.40 mm. A supplemental advantage is that the cutting resistance, an important property for steel cords lying in a protection ply, is increased with thicker filaments.